A data transmitting device such as a facsimile device is able to data-communicate with a destination facsimile device through a telephone transmission line.
However, in a facsimile communication, if an erroneous telephone number for a destination facsimile device is called because of erroneous actuations or operations, data is caused to be transmitted to the facsimile device having such erroneous telephone number so that secret information of this data is inadvertently informed to a third party. Also, in a facsimile communication, there is a possibility of a wire tapping to obtain data from a public telephone transmission line. These situations of the inadvertent erroneous transmission of a secret information and the wire tapping to obtain data occur not only in the facsimile communication but also in personal computer communication.
Not limited to the case where a public transmission line is used, in a communication system using a local area network (LAN), there is a problem that, in a facsimile communication of secret information between specific parties, a secret may leak to a third party by an inadvertent erroneous transmission, that is, a erroneous transmission or a wire tapping.
With regard to the problem caused by the erroneous actuations or operations, no effective means has yet been found, since the problem is caused by erroneous human actuations. With regard to the problem caused by the wire tapping, a ciphering / deciphering means has been proposed in which a secret key system, such as the data encryption standard (DES) system, and the public key system, has been used.
In the secret key system, since it is necessary to transmit a ciphered secret key from a transmitting station to a receiving station, there is a problem that if the ciphered secret key is obtained by a third party by wire tapping the transmission line, the ciphered data can be easily deciphered by the third party.
In the public key system, the transmission of a key for deciphering is unnecessary. However, since the public key for deciphering is open to the public, there is a problem that, once the name of the transmitting party is known, the ciphered data can be easily deciphered by the receiving station. In these secret key systems, there is also a problem that the data processing capacity is required to be increased for enhancing the secret ensuring ability (security).